The present invention relates generally to the field of medical imaging and in particular to the field of Computed Tomography (CT). Specifically, the invention relates to a technique for segmenting an acquired image data set and for removing obstructing structures, such as bone, from the acquired image.
CT imaging systems measure the attenuation of X-ray beams passed through a patient from numerous angles. Based upon these measurements, a computer is able to reconstruct images of the portions of a patient's body responsible for the radiation attenuation. As will be appreciated by those skilled in the art, these images are based upon separate examination of a series of angularly displaced cross sections. A CT system produces data that represent the distribution of linear attenuation coefficients of the scanned object. The data are then reconstructed to produce an image which is typically displayed on a computer workstation, and may be printed or reproduced on film. A virtual 3-D image may also be produced by a CT examination.
CT scanners operate by projecting fan-shaped or cone-shaped X-ray beams from an X-ray source. The beams are collimated and pass through the object, such as a patient, that is then detected by a set of detector elements. The detector element produces signals based on the attenuation of the X-ray beams, and the signals are processed to produce data that represent the line integrals of the attenuation coefficients of the object along the ray paths. These data or signals are typically called projections. By using reconstruction techniques, such as filtered backprojection, useful images are formulated from the projections. The locations of features of interest, such as pathologies, may then be located either automatically, such as by a computer assisted diagnosis algorithm or, more conventionally, by a trained radiologist.
The relative opacity of some structures, such as bone, to the X-rays employed in CT scanning may obstruct regions of interest from certain perspectives. For example, in CT angiography (CTA) the skeletal system may significantly hinder the visibility of critical vascular structures in the desired three-dimensional renderings. To address this problem, a structure or region mask, such as a bone mask, may be constructed. The mask may then be subtracted from the image, allowing the radiologist or technician to view the region of interest from the desired viewpoint without obstruction.
Construction of the structure mask, however, is not a trivial task and may require both complex algorithms as well as user intervention. This user intervention can lead to undesirable delays as well as to inter- and intra-user variability in the construction of the structure masks.
However various factors may complicate the fully automated construction of a structure mask by computer-implemented algorithm. For example, in the case of bone, overlapping image intensities, close proximity of imaged structures, and limited detector resolution may make the automated separation of structures difficult. In particular, the proximity of vascular structures and bone along the vertebra and near the pelvis make segmentation an exceedingly complex task for computer-based algorithms.
Other factors may also contribute to the problems associated with generating a mask by automated routine, such as the anatomic and pathological variability which exists in the patient population. Examples of such patient variability include the presence of vessels with calcified plaque deposits and the presence of interventional devices such as stents, both of which may confuse automated segmentation algorithms. These various factors contribute both to inadequacies in the structure masks which are derived and to a need for undesired and time-consuming human intervention in forming the masks. Among the benefits which may be realized by fully automating the formation of structure masks is the potential for real-time structure removal. Real-time structure removal may allow a technician to perform certain useful functions, such as to adjust the location or field of view of the scan, to optimize the bolus and timing of the introduction of the contrast agent, and to minimize the dose exposure to the patient.
There is a need therefore, for an improved technique for deriving a structure mask, such as a bone mask, preferably with little or no human intervention.